In western countries, coronary atherosclerotic heart disease (coronary heart disease) is the leading cause of death, and cholesterol is one of the risk factors that cause this disease. At present, there are two kinds of medicines that are used to the reduction of plasma cholesterol levels. One is Statins, which is a HMG-CoA reductase inhibitor, and can effectively inhibit the biosynthesis of cholesterol in vivo. Another kind of role is to prevent the absorption of cholesterol from small intestine, and Ezetimibe is a common cholesterol absorption inhibitor. U.S. Pat. No. 5,846,966 describes Ezetimibe, the chemical structure is as follows:

The side chain at the 3-position carbon of the azetidinone is a chiral benzyl alcohol, and the chiral carbon is of S configuration. The structure-activity relationship shows that the pharmacodynamic properties of S configuration is better than the R configuration, which indicates that the stereochemistry of the carbon of the benzyl is very important.
WO2011/017907 discloses a new kind of azetidinone compounds, which can also effectively inhibit the absorption of cholesterol, but the side chain on the 3-carbon of the azetidinone is not a chiral benzyl alcohol but an achiral allyl alcohol, and the pharmacodynamic properties of the double bonds of Z configuration is much better than the double bonds of E configuration. Among this kind of compounds, the chemical structure of the compound with the best pharmacodynamic properties, i.e. (3R,4S)-4-(4-hydroxyphenyl)-3-[3-(4-fluorophenyl)-4-hydroxybut-2(Z)-enyl]-1-(4-fluorophenyl)azetidin-2-one is as follows:

Because the synthetic route for the preparation of the new azetidinone compounds mentioned in WO2011/017907 is too long, and double bonds of Z configuration cannot be produced stereoselectively, some of the steps are not suitable for industrial production, therefore, it is necessary to develop a new process route. The present invention provides a new method for preparing this kind of azetidinone. The raw materials of the new process are easily obtained, and the process has a few synthetic steps, the double bond of Z configuration can be produced stereoselectively, and the operation is simple, the yield is high, the cost is low, and the process can be used for industrial production.